Multi-antenna wireless transmission technique, or Multiple Input Multiple Output (MIMO), can achieve spatial multiplex gain and spatial diversity gain by deploying a plurality of antennas at both the transmitter and the receiver and utilizing the spatial resources in wireless transmission. Researches on information theory have shown that the capacity of a MIMO system grows linearly with the minimum of the number of transmitting antennas and the number of receiving antennas.
FIG. 1 shows a schematic diagram of a MIMO system. As shown in FIG. 1, a plurality of antennas at the transmitter and a plurality of antennas at each of the receivers constitute a multi-antenna wireless channel containing spatial domain information. Further, Orthogonal Frequency Division Multiplexing (OFDM) technique has a strong anti-fading capability and high frequency utilization and is thus suitable for high speed data transmission in a multi-path and fading environment. The MIMO-OFDM technique, in which MIMO and OFDM are combined, has become a core technique for a new generation of mobile communication.
For instance, the 3rd Generation Partnership Project (3GPP) organization is an international organization in mobile communication field which plays an important role in standardization of 3G cellular communication technologies. Since the second half of the year 2004, the 3GPP organization has initiated a so-called Long Term Evolution (LTE) project for designing Evolved Universal Terrestrial Radio Access (EUTRA) and Evolved Universal Terrestrial Radio Access Network (EUTRAN). The MIMO-OFDM technique is employed in the downlink of the LTE system. In a conference held in Shenzhen, China in April 2008, the 3GPP organization started a discussion on the standardization of 4G cellular communication systems (currently referred to as LTE-A systems). Again, the MIMO-OFDM technique becomes a key technique for air interface in the LTE-A system.
In the LTE-A system, Carrier Aggregation (CA) is a new concept. FIG. 2 illustrates the CA concept in which a base station is provided with a plurality of downlink carriers and a plurality of uplink carriers. A number of carriers can be virtually combined into one carrier, which is referred to as carrier aggregation. The LTE-A system can support continuous CA as well as intra-band or inter-band non-continuous CA, with a maximum aggregated bandwidth of 100 MHz. In order to ensure effective utilization of the carriers at the initial stage of the commercial deployment of the LTE-A system, i.e., to ensure that LTE UEs can access the LTE-A system, each carrier should be configured to be backward compatible with the LTE system. However, it is also possible to design a carrier dedicated to the LTE-A system. At the research stage of the LTE-A system, related researches on CA focus on improvement of spectral utilization for continuous CA, design of control channels for asymmetric uplink/downlink CA scenario, and the like. Herein, the design of control signals involves feedback of downlink CSI from a UE to a BS.
There are two feedback channels for downlink CSI feedback, a Physical Uplink Control CHannel (PUCCH) and a Physical Uplink Shared CHannel (PUSCH). In general, the PUCCH is configured for transmission of synchronized, basic CSI with low payload; while PUSCH is configured for transmission of bursty, extended CSI with high payload. For the PUCCH, a complete CSI is composed of different feedback contents which are transmitted in different sub-frames. For the PUSCH, on the other hand, a complete CSI is transmitted within one sub-frame. Such design principles remains applicable in the LTE-A system.
The feedback contents can be divided into three categories: Channel Quality Index (CQI), Pre-coding Matrix Index (PMI) and Rank Index (RI), all of which are bit quantized feedbacks. In the LTE-A system, these three categories of contents are still the primary feedback contents. For PMI, it is currently agreed that a PMI is collectively determined from two pre-coding matrix indices, W1 and W2, where W1 represents broadband/long-term channel characteristics and W2 represents sub-band/short-term channel characteristics. In transmission of W1 and W2 over the PUCCH, it is not necessary for simultaneous feedback of W1 and W2 within the same sub-frame. Moreover, W1 or W2 may be omitted in the feedback. This is described in 3GPP R1-102579, “Way forward on Rel. 10 feedback”.
All the frequency areas corresponding to the CSI feedback are referred to as Set S. In the LTE system where there are only single-carrier situations, the Set S is defined as equal to the carrier bandwidth of the system. In the LTE-A system where there are additionally multi-carrier situations, the Set S can be defined as equal to the bandwidth of one single carrier or equal to the summed bandwidth of multiple carriers.
In the LTE system, the following eight types of MIMO transmission approaches for downlink data are defined:
1) Single antenna transmission. This is used for signal transmission at a single antenna BS. This approach is a special instance of MIMO system and can only transmit a single layer of data.
2) Transmission diversity. In a MIMO system, diversity effects of time and/or frequency can be utilized to transmit signals, so as to improve the reception quality of the signals. This approach can only transmit a single layer of data.
3) Open-loop space division multiplexing. This is a space division multiplexing without the need for PMI feedback from UE.
4) Closed-loop space division multiplexing. This is a space division multiplexing in which PMI feedback from UE is required.
5) Multi-user MIMO. There are multiple UEs simultaneously participating in the downlink communication of the MIMO system.
6) Closed-loop single layer pre-coding. Only one single layer of data is transmitted using the MIMO system. The PMI feedback from UE is required.
7) Beam forming transmission. The beam forming technique is employed in the MIMO system. A dedicated reference signal is used for data demodulation at UE. Only one single layer of data is transmitted using the MIMO system. The PMI feedback from UE is not required.
8) Two-layer beam forming transmission. The UE can be configured to feed back PMI and RI, or not to feed back PMI and RI.
In the LTE-A system, the above eight types of transmission approaches may be retained and/or canceled, and/or a new transmission approach, dynamic MIMO switching, can be added, by which the BS can dynamically adjust the MIMO mode in which the UE operates.
In order to support the above MIMO transmission approaches, a variety of CSI feedback modes are defined in the LTE system. Each MIMO transmission approach corresponds to a number of CSI feedback modes, as detailed in the following.
There are four CSI feedback modes for the PUCCH, Mode 1-0, Mode 1-1, Mode 2-0 and Mode 2-1. These modes are combination of four types of feedbacks, including:
1) Type 1: one preferred sub-band location in a Band Part (BP, which is a subset of the Set S and has its size dependent on the size of the Set S) and a CQI for the sub-band. The respective overheads are L bits for the sub-band location, 4 bits for the CQI of the first codeword and 3 bits for the CQI of the possible second codeword which is differentially coded with respect to the CQI of the first codeword.
2) Type 2: broadband CQI and PMI. The respective overheads are 4 bits for the CQI of the first codeword, 3 bits for the CQI of the possible second codeword which is differentially coded with respect to the CQI of the first codeword and 1, 2 or 4 bits for PMI depending on the antenna configuration at BS.
3) Type 3: RI. The overhead for RI is 1 bit for two antennas, or 2 bits for four antennas, depending on the antenna configuration at BS.
4) Type 4: broadband CQI. The overhead is constantly 4 bits.
The UE feeds back different information to the BS in correspondence with the above different types.
The Mode 1-0 is a combination of Type 3 and Type 4. That is, the feedbacks of Type 3 and Type 4 are carried out at different periods and/or with different sub-frame offsets. In the Mode 1-0, the broadband CQI of the first codeword in the Set S and possibly the RI information are fed back.
The Mode 1-1 is a combination of Type 3 and Type 2. That is, the feedbacks of Type 3 and Type 2 are carried out at different periods and/or with different sub-frame offsets. In the Mode 1-1, the broadband PMI of the Set S, the broadband CQIs for the individual codewords and possibly the RI information are fed back.
The Mode 2-0 is a combination of Type 3, Type 4 and Type 1. That is, the feedbacks of Type 3, Type 4 and Type 1 are carried out at different periods and/or with different sub-frame offsets. In the Mode 2-0, the broadband CQI of the first codeword in the Set S, possibly the RI information as well as one preferred sub-band location in the BP and the CQI for the sub-band are fed back.
The Mode 2-1 is a combination of Type 3, Type 2 and Type 1. That is, the feedbacks of Type 3, Type 2 and Type 1 are carried out at different periods and/or with different sub-frame offsets. In the Mode 2-1, the broadband PMI of the Set S, the broadband CQIs for the individual codewords and possibly the RI information, as well as one preferred sub-band location in the BP and the CQI for the sub-band are fed back.
There are thus the following correspondence between the MIMO transmission approaches and the CSI feedback modes:
MIMO transmission approach 1): Mode 1-0 and Mode 2-0;
MIMO transmission approach 2): Mode 1-0 and Mode 2-0;
MIMO transmission approach 3): Mode 1-0 and Mode 2-0;
MIMO transmission approach 4): Mode 1-1 and Mode 2-1;
MIMO transmission approach 5): Mode 1-1 and Mode 2-1;
MIMO transmission approach 6): Mode 1-1 and Mode 2-1;
MIMO transmission approach 7): Mode 1-0 and Mode 2-0;
MIMO transmission approach 8): Mode 1-1 and Mode 2-1, with PMI/RI feedback from UE; and
MIMO transmission approach 8): Mode 1-0 and Mode 2-0, without PMI/RI feedback from UE.
On the other hand, there are five CSI feedback modes for the PUSCH, Mode 1-2, Mode 3-0, Mode 3-1, Mode 2-0 and Mode 2-2.
In the Mode 1-2, the PMIs of the individual sub-bands in the Set S, the broadband CQIs of the individual sub-bands in the Set S and possibly the RI information are fed back.
In the Mode 3-0; the CQI for the first codeword of each, sub-band in the Set S, the broadband CQI of the first codeword in the Set S and possibly the RI information are fed back. Herein, the sub-band CQIs are differentially coded with respect to the broadband CQI, so as to reduce feedback overhead.
In the Mode 3-1, the CQIs for the individual codewords of each sub-band in the Set S, the broadband CQIs of the individual codewords in the Set S, the broadband PMI of the Set S and possibly the RI information are fed back. Herein, the sub-band CQIs are differentially coded with respect to the broadband CQIs, so as to reduce feedback overhead.
In the Mode 2-0, the locations of the preferred M sub-bands in the Set S, the broadband CQI for the first codeword in each of the M sub-bands, the broadband CQI of the first codeword in the Set S and possibly the RI information are fed back.
In the Mode 2-2, the locations of the preferred M sub-bands in the Set S, the broadband PMIs for the M sub-bands, the broadband CQIs for the individual codewords in each of the M sub-bands, the broadband PMI of the Set S, the broadband CQIs of the individual codewords in the Set S and possibly the RI information are fed back.
There are thus the following correspondence between the MIMO transmission approaches and the CSI feedback modes:
MIMO transmission approach 1): Mode 2-0 and Mode 2-0;
MIMO transmission approach 2): Mode 2-0 and Mode 3-0;
MIMO transmission approach 3): Mode 2-0 and Mode 3-0;
MIMO transmission approach 4): Mode 1-2, Mode 2-2 and Mode 3-1;
MIMO transmission approach 5): Mode 3-1;
MIMO transmission approach 6): Mode 1-2, Mode 2-2 and Mode 3-1;
MIMO transmission approach 7): Mode 2-0 and Mode 3-0;
MIMO transmission approach 8): Mode 1-2, Mode 2-2 and Mode 3-1, with PMI/RI feedback from UE; and
MIMO transmission approach 8): Mode 2-0 and Mode 3-0, without PMI/RI feedback from UE.
There are currently few references available for the CSI feedback in the LTE-A system, as this has not been discussed in the standardization process. The only existing documents mainly focus on the general design of the feedback, including:
1) Fundamental principle for designing CSI feedback. The periodic feedback can support at most five downlink carriers and utilizes a design principle similar to that specified in Release 8 so as to map onto one uplink carrier for feedback. In addition, it is necessary to consider how to reduce the feedback overhead and how to increase the payload of the feedback channel. However, this fundamental design principle fails to teach any specific implementation, which is thus still a technical gap to be filled. Reference can be made to 3GPP RAN1, “Final Report of 3GPP TSG RAN WG1#58bis v1.0.0”;
2) Frequency/time/spatial domain differential feedback. The UE performs differential process on the feedback information in frequency/time/spatial domain before feeding it back to the BS, so as to reduce feedback overhead. However, how to perform the differential feedback in an erroneous propagation and multi-carrier situation is still a problem to be researched. Reference can be made to 3GPPR1-101808, Intel Corporation, “Evaluation of enhanced MIMO feedbacks for LTE-A”;
3) Feedback based on multifold characterization. For spatial domain CSI, multiple feedbacks can be carried out by means of quantization characterization from various perspectives. Reference can be made to 3GPP, R1-102336, “Extending Rel-8/9 UE feedback for improved performance”, Qualcomm; and
4) Description of new problems on feedback over PUSCH and PUCCH. These problems include, among others, specific definitions for W1 and W2 in a two-codebook design; enhanced technique for feedback over non-periodic PUSCH; compression for feedback overhead. By far, there are no effective solutions for the above problems. Reference can be made to 3GPP, R1-102302, “CSI Feedback Enhancement for LTE-Advanced”, NTT DOCOMO.